The present invention relates generally to the determination of fracture orientation relative to a wellbore and relative to formation stress fields, and more specifically relates to such methods performed in response to analysis of pressures observed during a fracturing operation.
The use of test fracturing operations to determine reservoir or formation characteristics prior to the performance of a full scale fracturing operation is well-known. For example, the evaluation of a formation through use of a test fracturing operation performed through use of a fracturing fluid without proppant is well-known. Exemplary procedures of this type are those normally referred to in the industry as "minifrac" or "microfrac" operations.
As an example of a microfrac operation, during the microfrac operation a short interval of a wellbore is pressurized until a "breakdown" of the formation occurs. The formation will breakdown when the pressure at the formation reaches a "breakdown pressure," i.e., that pressure at which the tangential stress changes from compression into tension and reaches the tensile strength of the formation. At this point, the formation will yield to the stress, and a tensile fracture will be created. As pressure is monitored during the pressurization of the wellbore interval approaching the breakdown pressure, the characteristics of the monitored pressure curve will depend upon the fluid injection rate and the fluid leak-off rate. As pressure continues to be applied, the fracture will extend, and the extension pressure may either increase or decrease, depending upon any height restriction on fracture propagation and fluid leak-off. At some point, the injection will be ceased, and an instantaneous shut-in pressure will be recorded. As is known to the industry, this parameter will yield information regarding frictional pressure during the injection operation. Pressure decline after shut-in will be monitored, and the closure pressure will be determined. The closure pressure is that pressure at which the created fracture will close. This pressure will be equivalent to the minimum horizontal stress within the formation. If the shut-in is continued for an extended period, the formation will eventually reach an equilibrium pressure, at which time the pressure will be equal to the initial reservoir pressure.
Conventional minifrac and microfrac pressure analysis operations have not been capable of providing an indication of the direction of the fracture from the wellbore relative to stress fields existing in the formation. This information is highly desirable, as it will provide information useful, for example, in the design of future perforating operations and the design of full scale fracturing treatments for the wellbore. Additionally, the determination of a direction of fracture propagation, particularly in highly deviated or generally horizontal wells, may be particularly useful.
Accordingly, the present invention provides a new method and apparatus for utilizing observed pressure data during a test fracturing operation to determine the fracture direction relative to stress fields in the formation surrounding the wellbore. The method and apparatus of the present invention may also be particularly useful in deviated or horizontal wells to determine the direction of the fracture relative to the wellbore direction.